1. Field of the Invention
The present invention relates to an image processing apparatus that is mounted in a color copying machine or other equipment. More specifically, the present invention relates to an image processing apparatus that can improve reproducing quality of black letters or lines in a color image.
2. Description of the Prior Art
In the conventional image processing apparatus, in order to improve a reproducing quality of black letters or lines included in a color image, a portion of the black letters or lines in an original image is determined by area determining process, and an image processing such as edge emphasis is performed in accordance with the determination result. The image processing will be explained below.
First, the original image is read so as to obtain an input image signal, which passes a first order differential filter and a second order differential filter for extracting edge portions in the original image. In addition, low chroma areas of the input image signal are extracted. In accordance with the extraction result, black letters or lines of the original image and contours thereof are determined. It is determined whether a point is outside the edge (on the background) or inside the edge (on the black line) in accordance with the polarity of the output of the second order differential filter. Hereinafter, the outside of the edge is referred to as an outer black edge portion, while the inside of the edge is referred to as an inner black edge portion. Contours of the black letters or lines in the original image are recognized as inner black edge portions and outer black edge portions.
Next, in order to improve the reproducing quality of the black letters or lines, the following processes are performed for the inner black edge portions and outer black edge portions independently.
For pixels of the inner black edge portion, edge emphasizing process is performed, in which a lightness edge component (VMTF) is added to image data of a black component. For image data C (cyan), M (magenta) and Y (yellow) of color components, the edge emphasizing process is not performed, but another process is performed in which the image data of the referred pixel is replaced with the image data of the minimum value (i.e., the image data of the minimum density) in 5×5 or 3×3 pixel matrix.
For pixels of the outer black edge portion, the edge emphasis is not performed for any of image data K, C, M and Y of black and color components, but another process is performed in which the image data of the referred pixel is replaced with the image data of the minimum value in 5×5 or 3×3 pixel matrix.
The above-mentioned process suppress the color components C, M and Y in the neighborhood of the edges of the black letters or lines in the original image, and emphasize the inner black edge, so that the reproducing quality of the black letters or lines can be improved.
However, along with a recent high definition of the image reader, the conventional image processing explained above is not sufficient for suppressing color drift and improving the reproducing quality of the black letters or lines.
In an electrophotographic printer or other equipment, a color drift can be generated due to a misregistration of colors C, M, Y and K when overlaying the colors for printing. This color drift is relieved by the above-mentioned process that replaces the image data of the colors C, M and Y of the inner black edge with the minimum value in the matrix. When using a 5×5 pixel matrix for example, the densities of the colors C, M and Y are suppressed by the width of two pixels. Therefore, in the case of a printer that can print by a definition of 400 dpi for example, if a color drift (a misregistration between images C, M and Y) more than 128 microns that corresponds to the width of two pixels is generated, the effect of relieving the color drift by the above-mentioned process is reduced, and colors of C, M and Y can smears at the edge of the black letters or lines.
If the size of the image processing matrix (matrix size) is enlarged in accordance with a high definition, the above-mentioned process can be performed by a sufficient width for the color drift quantity, so that the problem of smearing of the colors C, M and Y at the edge of the black letters or lines can be solved. However, in this case, a capacity of memory necessary for storing the image data will also increase along with the increase of the matrix size, and the cost will increase substantially.